The present invention relates to a method of measuring the period with which a surface defect recurs along the length of a travelling web.
Various types of surface inspection apparatus for detecting surface defects of traveling webs, such as films, paper sheets, metal sheets, etc. are well known and widely used for quality control of the web.
Such a surface inspection apparatus is used to detect surface defects, such as scratches and pin holes, in a surface of a traveling web appearing not only at random but also periodically. Such periodic surface defect detection apparatus is known from, for example, Japanese Unexam. Patent Publ. No. 49-35091. According to such a periodic surface defect detection system, surface defects are counted by opening a time gate upon the lapse of a predetermined time period from the time the first surface defect is detected. After several repetitions of this periodic detection, if more than a predetermined number of surface defects are detected upon each periodic detection, the web is considered to have periodic surface defects.
In such periodic surface defect detection apparatus, because surface defects are counted for the predetermined time period starting at the time of a first detection of surface defect, if a surface defect that is detected first not a periodic surface defect, the period with which a surface defect appears periodically should be measured again. Therefore, it is hard to perform a real time measurement of the period with which a surface defect recurs in a surface of a web traveling along a web manufacturing line.
It has been proposed to perform the measurement of the period of recurrence of a surface defect in a real time manner by the use of an auto-correlation. To do this, a surface of a traveling web is divided into a plurality of data cells in the direction in which the web moves and each data cell is graded as defective or non-defective. The defective data cell is signified by a binary datum "1" and the non-defective data cell is signified by a binary datum "0". Then, the number of pairs of defects at a regular distance of j is calculated for a data row consisting of binary data for the number n of data cells by the use of the following auto-correlation: ##EQU1## wherein j is an integer from zero (0) to (n-1). In the auto-correlation I, (d.sub.i .multidot.di+j) is a product of a binary datum d.sub.i for an i-th data cell and a binary datum d.sub.i +j for a (i+j)-th data cell. That is, the product is "1" only when both of the binary data d.sub.i and d.sub.i+j are "1", indicating that both the i-th data cell and the (i+j)-th data cell are defective. Accordingly, the auto-correlation I expresses the number of pairs of defects at a regular distance of j for a data row consisting of binary data for the number n of data cells. Then, the following discrimination condition is used to determine the basic period of the surface defect: EQU AC(kf.sub.o)&gt;0 II
wherein k is an integer from 1 to l.
However, because of the necessity of calculating the auto-correlation I from all of the data, d.sub.i, d.sub.2, . . . , d.sub.n, the time required to perform the calculation is proportional to n.multidot.(n-1). That is, multiplication and addition have to be repeated n.multidot.(n-1) times for the number n of data. The number n is a value of the length L of the web undergoing testing divided by the unit length s.
The time necessary to perform the calculation of the period of a surface defect, which depends upon given hardware and program, has thus heretofore been long and a high processing capacity of the computer or special hardware is needed to perform such a calculation in an acceptably short time.